1. Field of the Invention
The present invention relates to image scanning systems and more specifically to systems and techniques for correcting color registration errors when a color image is scanned.
2. Description of the Related Art
Image scanners found in conventional color copying machines usually include three line-image sensors for detecting R, G, and B colors. These colors are typically detected along a main scanning direction and a sub scanning direction. In the sub scanning direction, the line image sensors are arranged such that each sensor is located or shifted by one or more pixel sizes away from the next. Theoretically, color registration errors due to the shifts in location of the three line image sensors can be accurately corrected by making delays corresponding to the shifts in location. On the other hand, in the main scanning direction, there is theoretically no shift of pixel location among the R, G, and B line sensors, and thus no color registration error occurs between pixels at the same location in the main scanning direction.
The line-to-line distance of the three-line image sensor is an integral multiple of the pixel size in the main scanning direction. Theoretically, therefore, the color registration errors due to the difference in location of three image sensors can be exactly corrected simply by delaying signals by an amount corresponding to the line-to-line distance using a line memory.
However, in practice, color registration errors can occur due to various other factors. For example, DC color registration errors may occur due to variations in optical paths caused by an optical factor associated with the lens. As another example, AC color registration errors can occur due to mechanical vibrations. In any event, the sum of the DC color registration error and the AC color registration error appears among R, G, and B.
Further yet, as another example, in a scanner that uses a reduction optical system, the reduction ratio is not completely equal among R, G, and B, which causes color registration errors to occur. The color registration error is smallest in the center of the main scanning direction and increases toward the periphery.
Various techniques have been proposed to correct such color registration errors (some techniques may be found, for example, in Japanese Patent Laid-Open Nos. 2000-22964, 2002-112046, and 10-42157).
In most conventional color copying machines, a document to be scanned is placed on a glass, and then light is scanned over the document to produce a color image. In this case, the document remains stationary or is maintained at a fixed position during the scanning operation. Hereinafter, this scanning mode will be referred to as a fixed scan mode.
In addition to the fixed scan mode, a flow-scan mode is also known. In the flow scan mode, although the scanning position is fixed, the document does not remain stationary, but rather continues to move during the scanning operation. Note that in the case of an automatic document feeder, each document is stopped at a scanning position on the glass while the scanning operation is performed. Compared with the fixed scan mode, the flow scan mode has various advantages in terms of cost, noise, power consumption, and weight.
However, the disadvantage of the flow scan mode is that because scanning is performed at a fixed scanning position while moving a document, scanning quality is sensitive to dust or stains on glass. This deficiency is overcome by using a high quality scanner.
In a color image scanner that can operate in both fixed scan and flow scan modes, a problem with correction of color registration errors occurs, as described below. First, a document is scanned by R, G, and B line photosensors one by one. The scan order for each of the fixed scan mode and the flow scan mode is opposite, so that the signs of correction values of color registration errors become opposite. Furthermore, the fixed scan mode and the flow scan mode have different optical paths, which results in a difference in absolute correction values. That is, if the same correction values are used to correct color registration errors in both the fixed scan mode and the flow scan mode, it is difficult to eliminate the influence of color registration errors on image data in both modes.
In the above-mentioned related art (Japanese Patent Laid-Open Nos. 2000-22964 and 2002-112046), the scanner has only correction values for correcting color registration errors in the fixed scan mode. In the scanner according to those patents, color registration errors are not properly corrected in the flow scan mode, and the improper correction causes an error in black character detection, which results in degradation in image quality.
Japanese Patent Laid-Open No. 10-42157 discloses an apparatus having the fixed scan mode and the flow scan mode but includes no description of correcting color registration errors in accordance with correction values separately prepared in the fixed scan and flow scan modes.
When an image, which includes picture or text is copied by a color copying machine (using YMCK colors) and the registration error among the YMCK colors of the printer unit is greater than about 0.1 mm, color bleeding becomes noticeable particularly in printed black character images.
To avoid the above problem, the color copying machine has a black character detector for separating a given document image into a text area and a picture area, whereby characters in a detected black text area are printed in black monochrome color with enhanced edges to improve the quality of the text image. To detect a black text area, the black character detector performs at least detection of chroma.
Although the color bleeding problem is resolved by using a black character detector, if the color registration error is greater than a certain value (for example, 0.5 pixels), the black character detector determines that any part of the image does not include a black character even if the image actually includes a black character.